1. Field of the Invention
The present invention relates to back side illuminated solid-state image sensors, and more particularly to small pixel size, back side illuminated CMOS image sensors having both global shutter (GS) and rolling shutter (RS) capabilities.
2. Discussion of Related Art
Typical image sensors sense light by converting impinging photons into electrons that are integrated (collected) in sensor pixels. After completion of an integration cycle collected charge is converted into a voltage, which is supplied to the output terminals of the sensor. In CMOS image sensors the charge to voltage conversion is accomplished directly in the pixels themselves and the analog pixel voltage is transferred to the output terminals through various pixel addressing and scanning schemes. The analog pixel voltage signal can also be converted on-chip to a digital equivalent before reaching the chip output. The pixels have incorporated in them a buffer amplifier, typically a Source Follower (SF), which drives the sense lines that are connected to the pixels by suitable addressing transistors. After charge to voltage conversion is completed and the resulting signal transferred out from the pixels, the pixels are reset in order to be ready for accumulation of new charge. In pixels that are using Floating Diffusion (FD) as the charge detection node, the reset is accomplished by momentarily turning on a reset transistor that conductively connects the FD node to a voltage reference, which is typically the pixel drain node. This step removes collected charge; however, it generates kTC-reset noise as is well known in the art. kTC noise has to be removed from the signal by the Correlated Double Sampling (CDS) signal processing technique in order to achieve the desired low noise performance. The typical CMOS sensors that utilize the CDS concept usually require four transistors (4T) in the pixel. An example of the 4T pixel circuit with pinned photodiode can be found in the U.S. Pat. No. 5,625,210 to Lee, which patent is herein incorporated by reference.
The principal disadvantage of standard CMOS sensors is that the pixel scanning, after charge has been accumulated in them, is performed in a sequential manner row by row. This generates an exposure time skew, which can be observed in the pictures of moving objects and which causes an undesirable picture distortion. This method of CMOS sensor scanning is called the “rolling shutter” mode and it resembles the action of the focal plane slit shutter in the old photographic film cameras. In most applications, however, it is preferable to expose all the pixels of the image at the same time without the skew and thus eliminate the distortion of moving objects. This type of sensor operation is called the “global shuttering” mode, which resembles the operation of a mechanical iris shutter in film cameras. In order to implement this kind of global shuttering it is necessary to provide another charge storage site in the pixels. After charge is integrated in the photodiodes of the pixels it is transferred to the pixel storage sites simultaneously in all the pixels of the array where it can wait for the scanning in the row by row fashion. The pixel scanning time skew is thus independent of the frame pixel exposure time. There have been several methods published in the literature of how to incorporate an additional charge storage site into the CMOS sensor pixels. A recent publication described in: ISSCC Digest of Technical Papers pp. 398, 399, by Keita Yasutomi, Shinya Itoh, Shoji Kawahito entitled: “A 2.7e Temporal Noise 99.7% Shutter Efficiency 92 dB Dynamic Range CMOS Image Sensor with Dual Global Shutter Pixels”, is a modification of the well known Interline Transfer CCD concept where charge from the pixel photodiodes is transferred first into vertical CCD registers located in the spaces between the pixels and then from there transferred in parallel fashion row by row into the serial register followed by the CCD transfer out into the common single amplifier. The application of the CCD charge transfer concept into the CMOS image sensor, to implement the global shuttering mode is shown in FIG. 1.
FIG. 1 represents a simplified circuit diagram of the prior art pixel 100 of a CMOS sensor that has the global shuttering capability. After charge integration is completed in the pinned photodiode 101 charge is transferred via the transfer gate transistor 103 into the second pinned photodiode 102 where it waits for scanning. The charge transfer from the first to the second pinned photodiode is completed in a CCD fashion without generating kTC noise. It is also necessary that the second pinned photodiode 102 have a higher pinning voltage than the first pinned photodiode 101 or the transfer gate 103 have a potential barrier and a well incorporated in it. Moreover it is necessary that the charge storage in the second pinned photodiode 102 be well shielded from impinging photons 115 to prevent undesirable smear effects when the objects in the scene move. The signal charge readout from the second pinned photodiode 102 then proceeds in the standard way by first resetting the Floating Diffusion (FD) node 104 to the drain bias voltage by momentarily turning on the reset transistor 106 followed by pulsing the gate of charge transfer transistor 105. This sequence can now proceed in a sequential order row by row. The signal appearing on the FD node 104 is buffered by the source follower transistor 107 which is addressed by a row addressing transistor 108. The signals to the charge transfer transistor gate, reset transistor, and the addressing transistor are supplied by the row buss lines 111, 112, 113 and 114 respectively. The Vdd bias is supplied to the pixels by the column Vdd line 109 and the signal output appears on the column output line 110. Using pinned photodiodes for charge storage is advantageous since it is well known that these photodiodes have a low dark current generation. High dark current in the storage sites would add to noise and also would generate undesirable shading effects in the picture that would have to be compensated for. Unfortunately, the second pinned photodiode 102 consumes a significant amount of valuable pixel area, thus increasing the size of the sensor and ultimately its cost. It is therefore desirable to investigate other possibilities of how to build CMOS sensors with the global shuttering capability that consume less pixel area but at the same time do not sacrifice pixel performance.